Known electroless plating pretreatment agents include plating pretreatment agents for catalyst adhesion, which adhere Pd or another precious metal to the surface of an object to be plated in order to confer electroless plating activity. Adhesion of the catalytic precious metal to the surface to be plated can be accomplished by a method such as coating with a solution of the pretreatment agent, dipping the object in a solution of the pretreatment agent, or drawing on the surface to be plated with an (inkjet) ink. The pretreatment agent must be one that can be used as a stable solution in order to achieve smooth adhesion to the object to be plated and uniformity of subsequent electroless plating.
Conventionally, colloidal solutions of tin and palladium and aqueous solutions of palladium chloride and other palladium compounds have been used as electroless plating catalysts. However, with aqueous solutions of inorganic palladium compounds for example one problem is that a sufficient quantity of Pd may not be retained because wettability is poor with respect to resin substrates that do not have affinity to aqueous solutions, or because the adhered Pd is removed by subsequent water washing, so organic solvent solutions are preferred as pretreatment agents for non-hydrophilic resin substrates. However, when pretreatment agents are provided as organic solvent solutions, there is a problem that the solubility is poor when inorganic palladium compounds are dissolved in the organic solvents used to dissolve resins and other organic compounds, so the palladium precipitates and a uniform solution cannot be obtained. Palladium acetate with its lower fatty acid is soluble in methanol at some concentrations, but the problem is that the palladium soon settles.
As an electroless plating pretreatment agent that is soluble and stable in inorganic solvents, Patent Document 1 discloses a pretreatment agent for electroless plating using a metal soap. It also discloses adding a silane coupling agent having a metal-capturing capability, but in both cases the pretreatment solution may turn black by gradual reduction of yellow Pd(II) into Pd(0), and a stable bivalent state of palladium cannot be maintained. Pd(II) can be stabilized by adding the aforementioned silane coupling agent, but if enough of the silane coupling agent is added to stabilize the Pd(II), the silane coupling agent covers the Pd because it has large molecules and a strong ability to coordinate to the Pd, making it difficult to reduce Pd(II) into Pd(0) having a capability of activating electroless plating after pretreatment of the object to be plated, and detracting from the subsequent plating properties in some cases.
Pd(II) cannot be thoroughly stabilized by such conventional methods, and while catalytic activity increases as Pd(II) is gradually reduced into Pd(0), the catalytic activity changes over time, and sediments may occur during long-term storage. If the Pd(II) is stabilized, on the other hand, it may then be difficult to reduce into Pd(0) as described above, and problems with the subsequent electroless plating properties may occur in some cases.